For more than ten years, continuous casting process has been actively developed and improved by several major steel companies. In continuous casting operations, molten metal such as steel, aluminium, copper, solidifies against the mold walls while it is simultaneously withdrawn from the bottom of the mold. Equipment used in casting operations must possess properties include thermal shock resistance, stability at high temperature, and ease of machinability—all lending themselves to the use of refractory-based components.
In the continuous casting process, the molten metal comes into contact with various refractory material components wherein the solidification of the cast metal product is initiated, including but not limited to the nozzles introducing the liquid metal into the ingot mold, the casing of the thermocouple being in contact with the molten metal, the slidegate plates and stopper rods used to control the flow of molten metal, and the side walls which ensure the confinement of the liquid metal between the cooled surfaces of the twin rolls. The side walls are also called side dams, end dams, edge dams, or break rings in horizontal casting operations.
In an example of the strip casting process, molten metal is introduced between two nickel-coated copper rolls with the side walls being in continuous contact with the rolls containing the molten metal. In additional to the chemical interaction between the side dam components and the molten or partially solidified metal, the side dam refractory components are subject to high stresses due to localized temperature differences of about 1 500–2000° C. to roll contact and back plate temperature of about 200–500° C. Strip casting typically requires prolonged contact with molten metals—much longer than typically unexpected in continuous casting operations, of at least 3 hours or more. Therefore, besides the tensile stresses developed in the roll contact areas in a typical strip casting process, the chemical interaction intensifies in strip casting as compared to continuous casting with the long processing time and the length of the strip produced in strip casting operations (several miles), thus leading to frequent and localized material erosion on the refractory equipment. Therefore, it is imperative in strip casting operations for the refractory side dams to have a minimum volume loss and sufficient strength to provide a seal between the rollers and the molten metal and last through a casting operation, while having sufficient lubricating properties and being sufficient weak as not to damage the surface of the coating on the rollers.
The life of the refractory-based components is determined by the mechanical and chemical wear in continuous exposure to molten metal, or by process parameters such as the batch size of the melt to be cast or the casting conditions. At the end of the life of the component, it is typical of the refractory equipment volume to be mostly eroded away either mechanically or chemically. Besides the refractory equipment itself, it is also typical for the coating of the rollers in contact with the refractory side dams to be damaged/worn away in a casting cycle.
Published patent application WO9932246 discloses an apparatus for lubricating edge dams in twin-roll strip casting machine, wherein the edge dams are made of a refractory based material comprising a silicon nitride/boron nitride (Si3N4—BN) composite, an aluminium nitride/boron nitride (AlN—BN) composite, or a silicon carbide/boron nitride (SiC—BN) composite.
To solve the problem of the deterioration of the refractory equipment in casting operations, U.S. Pat. No. 6,051,058 discloses a protective aqueous layer of about a few 1/10 mm to a few mm thick as a coating for refractory material members that are used in casting operations. The aqueous coating comprises a dispersion in an aqueous solvent of 20–50 wt. % of hexagonal boron nitride and at least one metal oxide selected from the group consisting of zircon, zirconia, alumina, and silica.
Applicants have found that it is not necessary to apply a protective coating layer on refractory equipment as in the prior art. Our invention relates to refractive side dams comprising a ceramic composite which forms a reaction/coating layer on the surface of refractory equipment that surprisingly and substantially prolongs the life of the refractory-based side dams used in casting operations, while exhibiting excellent tribological properties in friction environments, i.e., minimizing the wear and damages on the rollers being in contact with the refractory side dams in casting operations.